At a time when consumable energy sources are being depleted and becoming more expensive, there is increased motivation to inexpensively harness solar energy.
One type of solar generating plant comprises an array of posts located in a field and holding panels of solar devices. Typically, these panels track the sun across the sky, maintaining an angle which presents maximum solar radiation to the devices. Optical means for concentrating solar power onto a small solar cell are generally included thus providing more power for a given solar cell area.
One concentrator which has proven practical in protecting delicate solar cells as well as concentrating large amounts of solar power onto the surfaces of the cells is shown in FIG. 1. Post 11 is rigidly attached to the ground and extends vertically upward. At the top of post 11 is attached long bar 12, held by motorized gear box 15 which is controlled to track the sun across the sky. Attached to long bar 12 are cross bars 13a-13h. To each of cross bars 13a-13h are attached two tubs 14a-14h and14a'-14h' at opposite sides of the mid-point where the cross bars 13a-13h are attached to the long bar 12. For clarity only tubs 14h and 14h' are shown. Each tub has within it an array of focusing devices, each of which focuses sunlight onto a solar cell located at the bottom of the tub.
FIG. 2 shows tub 14a in a more normal detail. Located on the top surface of tub 14a is an array of Fresnel lenses 21a-21j. The focal points of these lenses are at approximately the bottom surface of tub 14a. Located in the vicinity of each focal point is an associated solar cell 24 a-24j. Solar cells 24b, 24c, 24g, and 24h are shown in FIG. 2 but only solar cell 24g is numbered in FIG. 2. Each of the solar cells 24a-24j is supported by a metal and ceramic heat sink 23a-23j (only heat sinks 23g and 23h are numbered in FIG. 2) which is attached to the bottom of tub 14a. Attached to solar cells 24a-24j are secondary optical elements 22a-22j (only elements 22b, 22 c and 22g are numbered in FIG. 2) each having four walls which reflect solar radiation onto cells 24a -24j respectively, thus further concentrating the solar energy transmitted through Fresnel lenses 21a-21j onto solar cells 24a-24j and making orientation of the array with respect to the sun less critical than would be the case without these walls.
Early solar cells had an efficiency of converting solar radiation energy into electricity of less than 10%, and were not economically feasible as a major power source.
A more efficient design, called a point contact solar cell, is described in Swanson, U.S. Pat. No. 4,234,352, especially with respect to Swanson FIGS. 5-10. This Swanson patent is incorporated herein by reference. The point contact cell is further described by Ronald A. Sinton in his doctoral dissertation entitled "Device Physics and Characterization of Silicon Point-Contact Solar Cells", Stanford University, 1987, particularly at pages 18-22 , also incorporated herein by reference. The point-contact cell provides an array of point contacts for collecting electrons and holes which have been separated by impinging photons from the sun. As further discussed by R. A. Sinton, Young Kwark, J. Y. Gan and Richard M. Swanson in an article "27.5-Percent Silicon Concentrator Solar Cells", IEEE Electron Device Letters, Vol. EDL-7, No. 10, Oct. 1986, also incorporated herein by reference, this device achieves an efficiency approaching 28%, more than twice that of previous silicon solar cell structures.
FIG. 3a shows an exploded view of a prior art-point contact solar cell. A piece of semiconductive silicon crystal 31 has formed within it and contacting its bottom surface, wells of p-type and n-type impurities such as wells 31p1, 31n1, 31p2, and 31n2, where the letter "p" or "n" at the end of each number 31 represents the conductivity type of the designated well The pattern of conductivity types of the impurity wells such as wells 31p1, 31n1, 31p2, and 31n2alternates throughout the bottom surface of semiconductor crystal 31. These impurity wells collect the charged particles generated by solar photons. In order to produce a current, an electrical path to outside circuitry must be provided.
One electrical path configuration, not shown, is to provide interleaved metal fingers arranged so that wells of one conductivity type contact one set of fingers and wells of the other conductivity type contact the other set of fingers. These sets of fingers are then connected to bus bars which carry the current away from the cell. Such an arrangement suffers a loss of efficiency due to the ohmic resistance of the long thin fingers.
Another electrical path geometry, shown in exploded view in FIG. 3a and in cross-section in FIG. 3b, overcomes the ohmic loss inherent in the long thin fingers by having thin fingers of short length. FIG. 3b shows a cross section taken along the lines B--B of FIG. 3a.
The structure of FIGS. 3a and 3b is formed as an integrated circuit having a 2-layer metal interconnect structure. As shown in FIGS. 3a and 3b, the cell is oriented so that the metal layers are on the bottom, rather than the top of the structure. Crystal 31 is the semiconductor substrate, passivation layer 35 is oxide (typically an oxide of silicon) adjacent the substrate, metal layer 32 is the first metal interconnect of an integrated circuit structure, glass layer 33 is a passivation layer between first and second metal, and metal layer 34 is the second metal interconnect of an integrated circuit structure.
Beneath and adjacent crystal 31 is passivation layer 35 patterned to have openings such as 35o1 to expose the p and n wells such as wells 31n1, 31p1, 31n2, and 31p2. Contacting passivation layer 35 is n-contact metal layer 32, having openings such as 31o2, surrounding small conductive p-well pads such as 32p1, 32p2. The openings such as openings 32o1, 32o2 in metal layer 32 are positioned so that when conductive protrusions or "fingers" such as protrusions 32n1 and 32n2 of n-contact metal layer 32 are in electrical contact with semiconductor crystal 31 through openings such as 35o1 and 35o3 (but not 35o2), respectively, in insulation layer 35, the p-wells in crystal 31 are not contacted by any part of n-contact layer 32. Rather, only the n-wells such as 31n1 and 31n2 in crystal 31 are contacted by the layer 32. The p-wells such as wells 31p1, 31p2 of semiconductor crystal 31 are contacted only by the small conductive pads such as pads 32p1, 32p2 formed in the center of openings 32o1 and 32o2, respectively.
Insulating glass layer 33 contacts the bottom surface of n-contact metal layer 32 and has insulating protrusions such as 33r1 and 33r2 which extend through openings such as 32o1, 32o2 of n-contact metal layer 32. Protrusions such as 33r1 and 33r2 surround the small conductive pads such as 32p1 and 32p2. Protrusions such as 33r1 and 33r2 contact passivation layer 35, thus providing electrical insulation between n-contact conductive layer 32 and the p-wells such as 31p1, 31p2 of semiconductor crystal 31. Vias such as 33o1, 33o2 are formed through the centers of protrusions such as 33r1 and 33r2, respectively, from insulation 33. The p-contact conductive pads such as 32p1, 32p2 are partly located in vias 33o1 and 33o2 and extend to and contact p-wells 31p1 and 31p2. P-contact metal layer 34 is located beneath insulating glass layer 33 and includes short protrusions or "fingers" such as 34p1, 34p2 which extend part way through vias such as 33o1, 33o2 in insulating layer 33 and contact the small conductive pads such as 32p1, 32p2, which in turn contact the p-wells such as 31p1, 31p2 in semiconductor crystal 31. Thus metal layers 32 and 34 gather the opposite charges generated in semiconductor crystal 31 by solar radiation incident on crystal 31.
FIG. 4a shows a geometry for packaging the cell of FIGS. 3a and 3b and bringing out metal leads to connect metal layers 32 and 34 of FIGS. 3a and 3b to outside wiring. As shown in FIG. 4a, cell 24g has p-wells and n-wells (such as wells 31n1, 31p1 shown in FIGS. 3a and 3b) embedded into only its center portion 31a. Secondary optical element 22g is attached to the solar cell at the outer edges of active region 31a so that all sunlight is focused onto active area 31a. P-contact metal layer 34 is positioned beneath this active area 31a. In contact with metal layer 34 is portion 44b of p-contact bus 44 which includes a side extension 44a leading to outside wiring (not shown). N-contact metal layer 32 extends to the perimeter of cell 24g and region 32e (also shown in FIGS. 3a and 3b) extends downward near the perimeter of cell 24g into the plane of p-contact layer 34. N-contact layer 32 is separated at all points from p-contact layer 34 by insulating glass layer 33, shown in FIGS. 3a and 3b and discussed above. Contacting the downward extending perimeter 32e of n-contact layer 32 is portion 42b (FIG. 4a) of n-contact bus bar 42 leading to outside wiring. Copper (or other metal) heat spreader 23g (also shown in FIG. 2) attaches to back plane heat sink 16a at the bottom of tub 14a.
With the packaging of FIG. 4a, not all of solar cell 24g can be used for actively collecting energy. This is because a perimeter including n-contact extension 32e must be provided to connect to bus bar 42b. Since n-contact region 34 does not extend to the perimeter, no electricity can be generated in this perimeter area.
Cell 24g of FIG. 4a, in addition to not making use of the full surface area of semiconductor crystal 31, also requires attaching secondary optical element 22g directly to crystal 31 in order to maintain the high conversion efficiency of 28%. Delicate semiconductor crystal 31 is not well adapted to reliably support such a structure.
According to an interim report entitled "High-Concentration Photovoltaic Module Design" by Black & Veatch, Engineers-Architects, prepared for Electric Power Research Institute, Aug. 1986, and available from Research Reports Center, Box 50490, Palo Alto, CA 94303 as Report No. AP-4752, as discussed on pages s-9 to s-10, test cells similar to cell 24g shown in FIG. 4a, in which bus bars similar to bus bars 42 and 44 were formed of a sandwich having an insulating alumina core with direct bonded copper on both top and bottom surfaces, and which had heat spreaders of nickel plated aluminum, were cycled in temperature from -30.degree. C. to +150.degree. C. at one cycle per 80 minutes, and all samples failed between 200 and 350 cycles due to separation of the solder bond between the copper metallized alumina layer (42b and 44b in FIG. 4a) and a nickel plated aluminum heat spreader (23g). Failure was attributed to voids associated with solder dewetting of a gold-covered nickel plating on the substrate lower copper foil.
An alternative approach that eliminates insulation above the central portion of the heat spreader is shown in FIG. 4b. As shownin FIG. 4b, bus 44 in FIG. 4a is eliminated and insulator 45 has a hole in its center, thereby insulating only bus 42 from heat spreader 23g. Bus 44 is replaced by a solder plug 46, which extends between a central portion of heat spreader 23g and p-contact metal 34 at the bottom surface of cell 24 g. Heat spreader 23g also serves as the contact between a positive conductor and the cell. A cell of this type is described by Black & Veatch on p. s-10 of the above cited paper and shown in FIG. 4c. Components in FIG. 4c equivalent to components in FIG. 4b have the same numbers. As stated by Black & Veatch, the structure of FIG. 4c reduces thermal resistance, allowing the cell to operate at a cooler temperature, and also eliminates the solder bond between substrate and heat spreader which caused the earlier failure. However, a relatively large mismatch in coefficients of thermal expansion exists between the silicon solar cell and the copper heat spreader.
The cell of FIG. 4b still suffers from the nonuse of the peripheral portion of 32e of the solar cell where the n-contact layer 32 extends down to contact bus bar 42. The cell of FIG. 4b also retains the structural weakness from attaching secondary optical element 22g directly to the top surface of delicate solar cell 24g at the rim of the active area.